There are presently two categories of electromyography recordings. The first is a non-invasive surface electromyography (sEMG) in which adhesive bio-electrodes are placed on the surface of the skin directly above the muscle under investigation. This category is accordingly limited in application to superficial muscles that are close to the skin's surface.
The second category is an invasive procedure for obtaining electromyography recordings that utilises fine-wires or needles that physically pierce the skin in order to communicate with a so-called deep muscle that is under investigation. Not only is this technique invasive, but some deep muscles that are located close to major blood vessels, nerves, and viscera should not be investigated at all by such a technique for patient safety reasons. Such established invasive EMG techniques are described in a scientific review article by Jasper R Daube and Devon I Rubin (2009) entitled “Needle electromyography” in Muscle & Nerve 39, no. 2: 244-270.
Invasive electromyography carries with it a number of risks such as the potential for infliction of additional trauma on a patient as well as that of infection. Such risks thus preclude its routine use in sports performance testing and training, particularly for elite athletes who may risk a performance-reducing or livelihood-destroying injury or infection as a result of such a procedure.
There have been previous attempts to investigate deep muscles non-invasively. In international patent publication number WO 2004107976 there is described a method and device for assessing the function of a deep muscle of a subject in which a patient table, force transducers and an ultrasound device are employed for visualising the deep muscle of interest. A computer may be connected to the transducers and ultrasound device to provide cues to the subject during the conduct of an assessment session and to analyse information obtained from the session. The device may also include one or more slings for supporting a limb of the subject.
In U.S. Pat. No. 6,185,451 there is described a method and apparatus for assessing the function of deep joint stabilizing muscles in which superficial muscles are monitored using electromyography during performance of an activity known to require recruitment, primarily, of deep stabilizing muscles when performed correctly. If the deep muscle functions adequately, there is little activity of the superficial muscles. Conversely, if the deep muscle function is inadequate, the superficial muscle activity is increased. Monitoring of the superficial muscles using electromyography may be combined with monitoring of the deep joint stabilizing muscle using ultrasound imaging and/or pressure biofeedback. The apparatus includes a surface electromyography unit, an ultrasound unit, a pressure biofeedback unit and vitalograph, in combination with a computer programmed to analyze data from them and given an indication of function.
The sEMG of five specific deep muscles may be recorded on the surface using conventional sEMG methods, with a 15-20% error, as demonstrated by McGill et al (1996, J. Biomechanics, 29(11), p 1503-1507) who recorded psoas, quadratus lumborum, external & internal obliques and transversus abdominus muscles. However this is only applicable to those specific muscles and not generalizable to all deep muscles.
Jesinger & Stonic (1994, IEEE DSP Workshop Proc., p 57-60) proposed an inverse finite element modeling (FEM) method to resolve superficial from deep muscle activity. However this method requires a priori information such as a multiple-slice MRI image of the body segment. Such inverse FEM models do not provide unique mathematical solutions unless there are a large number (>>100) of measurement points, and despite being introduced in 1996, to date there is no clinical evidence that this method is practically able to resolve deep from superficial muscle activity. Using the same approach, now termed ‘computed myography’, but with a number of mathematical approximations, van den Doel et al (2008, Inverse Problems, 24, p 1-17) attempted to resolve biceps, brachialis, and triceps activity but was only successful in demonstrating biceps and triceps activity resolution since their posited brachialis component behaved identically to biceps. Triceps and biceps are however both antagonistic superficial muscles that are easily recorded and differentiated using standard sEMG anyway. This inverse FEM method therefore appears to be more suited to academic computer simulated experiments as opposed to a being a viable clinical investigation tool.
In summary, as far as applicants are aware, aside from five specific deep muscles (McGill et al, 1996), deep muscle activity may currently only be actually recorded using the already established invasive electromyography techniques, and the two hypothesised techniques suggested by the non-invasive patents involving relating deep muscle activity to ultrasound imaging of changes in width and indirect inference of deep muscle activity by monitoring co-agonist superficial muscle EMG as outlined above.